1. Field of the Invention
This invention relates to a heat roller used, for example, to fix a toner image, in an electrophotographic copier, a laser printer or a fax machine and the like.
2. Description of Related Art
Conventionally, in an electrophotographic copier and the like, a heat roller system is widely used to fix a toner image formed on a recording material by heating. In the heat roller system, the non-fixed toner image is fixed to a recording material by passing the recording material with the non-fixed toner image adhered to it between a heat roller and a press roller located opposite and in contact with the heat roller.
Such a heat roller of the heat roller system as described above has a heating lamp installed inside a cylindrical metallic substrate acting as a roller component, and the metallic substrate is heated up to a predetermined temperature by heat generated from the heating lamp so as to cause the non-fixed toner image to be heated and fixed to the recording material.
In recent years, it has become necessary in the heating and fixing device of the heat roller system that the surface temperature of the heat roller reaches a suitable temperature (at which heating and fixing can be carried out) within a short period of time after turning on a main switch of the device and that the time in which this temperature is reached (hereinafter called xe2x80x9cstart-up timexe2x80x9d) is shortened to an order of seconds.
It has been proposed to reduce the start-up time to an order of seconds by making the cylindrical metallic substrate thin although this resulted in the problem that the possibility of reducing the wall thickness is limited, and in the case that the wall thickness is quite thin the strength of the metallic substrate is decreased.
Further, although the distance between the outer surface of the bulb of the heating lamp installed inside the metallic substrate and the inner surface of the metallic substrate was made small, i.e. the inner diameter of the metallic substrate was reduced, the specific design requirements for certain heating devices limit the applicability of this measure to only some kinds of heating devices.
The above-described studies exclusively consider the components of the heat roller other than the heating lamp, and the heating lamp itself acting as a heating source was not studied.
The present inventors have now investigated the heating lamp itself and studied the following items.
The principle of generating heat by a heating lamp consists in that an electric energy is supplied to a filament installed in a heating lamp, to increase the temperature of the filament and to generate heat in the heating lamp by thermal energy being radiated from the high temperature filament.
Just after the main switch of the device is turned on, the thermal energy radiated from the filament is absorbed by the encapsulated gas which is present around the filament and contains halogen and rare gas and thus heats up the gas. Further, the bulb is heated by the enclosed high temperature gas. In other words, it has been found that the absorption phenomenon reduces that part of the thermal energy radiated from the filament which passes through the gas and the bulb to directly reach the metallic substrate and which is not absorbed by the enclosed gas, and this is the cause for delaying the speed with which the metallic substrate increases its temperature.
Further, as a result of the inventor""s investigations, it has been found that the extent to which thermal energy radiated from the filament is absorbed by the enclosed gas is substantially influenced by the thermal conductivity of the enclosed gas.
In the prior art heating lamps, argon was mainly utilized as the rare gas and a small amount of halogen was also enclosed. The large amounts of argon in the enclosed gas determined the thermal conductivity.
As a result, since the thermal conductivity of argon is quite high, namely 177xc3x9710xe2x88x924 (W/m.K), a certain percentage of the total thermal energy radiated from the filament is absorbed by the argon present around the filament. Consequently, it has been found that the thermal energy radiated from the filament is not efficiently transmitted directly to the metallic substrate and the temperature increasing speed in the metallic substrate cannot be made fast.
The present invention was made to solve the aforesaid problems. Particularly, it is an object of the present invention to devise a heat roller having a short start-up time while using a heating lamp which is provided with electric energy which is not higher than in the prior art devices and wherein the thermal conductivity of the gas encapsulated in the heating lamp is reduced.
The heating lamp used in the heat roller of the present invention reduces the amount of thermal energy absorbed by the enclosed gas present around the filament as compared with that of the prior art heating lamps even though the total thermal energy radiated from the filament is the same as in the prior art. Almost immediately after the main switch of the device is turned on, a high percentage of the thermal energy radiated from the filament can directly and efficiently be transmitted to the metallic substrate, and the temperature increasing speed of the metallic substrate is fast. That is, the present invention provides a heat roller enabling the surface temperature of the heat roller to reach a suitable operation temperature within a short period of time.
The heat roller described in a first embodiment of the present invention is comprised of a cylindrical metallic substrate and a heating lamp installed axially inside the metallic substrate. The heating lamp has a filament installed in the bulb, gas is enclosed in the bulb and the thermal conductivity of the enclosed gas is 110xc3x9710xe2x88x924 (W/m.K) or less at room temperature, for example, 25xc2x0 C.
Setting the thermal conductivity of the enclosed gas of the heating lamp at 110xc3x9710xe2x88x924 (W/m.K) or less enables the heating lamp used in the heat roller of the present invention to reduce the amount of thermal energy which is absorbed by the enclosed gas present around the filament as compared with the prior art heating lamps even if the same electric energy as in the prior art is applied to the filament and also the total thermal energy radiated from the filament is the same as in the prior art. It is thus possible to transmit the thermal energy radiated from the filament at a quite high rate efficiently and directly to the metallic substrate almost immediately after the main switch of the device has been turned on, and there is provided a heat roller in which the temperature increasing speed of the metallic substrate can be increased and the desired surface temperature of the heat roller can be reached within a short period of time.